Hydration of high-volume fly ash cement pastes

The hydration processes of high-volume fly ash cement paste were investigated by examining the non-evaporable water content, the CH content, the pH of pore solution and the fraction of reacted fly ash, curing at either 20°C or elevated temperatures after an initial curing at 20°C. The replacement percentage levels of fly ash were 40%, 50% and 60% by weight, respectively. The results revealed that the non-evaporable water content in high-volume fly ash cement pastes does not develop as plain cement pastes does, so it may be improper to apply the non-evaporable water content to evaluate the hydration process in high-volume fly ash cement matrix. The reduction in CH content increases with the progressing of hydration process and varies linearly with the logarithm of curing age. The addition of 3.0% of Na₂SO₄ could accelerate the pozzolanic reaction of fly ash at early ages. At 20°C, the pH of pore solution of high-volume fly ash cement paste was reduced to a great extent at early ages and it continued to decline at later ages due to the inclusion of large amount of fly ashes. At elevated temperatures, however, this trend was not found. The fraction of reacted fly ash directly reflects the pozzolanic reactivity of fly ash both at normal and elevated temperatures. There is some inherent correlation between the reduction in CH content, the pH of pore solution and the fraction of reacted fly ash. For specified matrix, the consumption of CH and the pH of pore solutions change linearly with the increase of the fraction of reacted fly ash.     

Effect of water curing conditions on the hydration degree and compressive strengths of fly ash–cement paste

This paper explains the effect of water curing condition on compressive strengths of fly ash–cement paste by quantitative data of hydration degree. Hydration of fly ash–cement paste was estimated by Rietveld analysis and selective dissolution. The result shows that the hydration degree of belite is affected by water curing conditions, more so than that of fly ash and alite. Fly ash still continues to hydrate even without an extra, external supply of water. The strong dependence of fly ash–cement concrete on curing conditions does not come from the hydration degree of fly ash, but rather comes from the hydration degree of cement, especially belite. When the water to binder ratio is low enough, the hydration of cement plus small hydration of fly ash are considered to be enough for adequate compressive strength at the beginning. Then, compressive strength of fly ash–cement paste becomes less sensitive to the water curing period.     

Strength of lime–fly ash compacts using different curing techniques and gypsum additive

Lime–fly ash mixtures are exploited for the manufacture of fly ash bricks finding applications in load bearing masonry. Lime–pozzolana reactions take place at a slow pace under ambient temperature conditions and hence very long curing durations are required to achieve meaningful strength values. The present investigation examines the improvements in strength development in lime–fly ash compacts through low temperature steam curing and use of additives like gypsum. Results of density–strength–moulding water content relationships, influence of lime–fly ash ratio, steam curing and role of gypsum on strength development, and characteristics of compacted lime–fly ash–gypsum bricks have been discussed. The test results reveal that (a) strength increases with increase in density irrespective of lime content, type of curing and moulding water content, (b) optimum lime–fly ash ratio yielding maximum strength is about 0.75 in the normal curing conditions, (c) 24 h of steam curing (at 80°C) is sufficient to achieve nearly possible maximum strength, (d) optimum gypsum content yielding maximum compressive strength is at 2%, (e) with gypsum additive it is possible to obtain lime–fly ash bricks or blocks having sufficient strength (>10 MPa) at 28 days of normal wet burlap curing.     

Effect of fly ash fineness on compressive strength and pore size of blended cement paste

This paper presents an experimental investigation on the effect of fly ash fineness on compressive strength, porosity, and pore size distribution of hardened cement pastes. Class F fly ash with two fineness, an original fly ash and a classified fly ash, with median particle size of 19.1 and 6.4 μm respectively were used to partially replace portland cement at 0%, 20%, and 40% by weight. The water to binder ratio (w/b) of 0.35 was used for all the blended cement paste mixes.Test results indicated that the blended cement paste with classified fly ash produced paste with higher compressive strength than that with original fly ash. The porosity and pore size of blended cement paste was significantly affected by the replacement of fly ash and its fineness. The replacement of portland cement by original fly ash increased the porosity but decreased the average pore size of the paste. The measured gel porosity (5.7–10 nm) increased with an increase in the fly ash content. The incorporation of classified fly ash decreased the porosity and average pore size of the paste as compared to that with ordinary fly ash. The total porosity and capillary pores decreased while the gel pore increased as a result of the addition of finer fly ash at all replacement levels.     

碱式硫酸镁水泥耐酸腐蚀性能研究

为了研究碱式硫酸镁水泥耐酸腐蚀性能,将不同配比的水泥试样分别在柠檬酸溶液及水中浸泡不同龄期,再进行质量变化测定及抗折强度和抗压强度试验。采用XRD与SEM技术分析不同配比水泥试样浸泡于两种溶液后的物相组成和显微形貌。结果表明,掺入的矿渣和粉煤灰对碱式硫酸镁水泥具有良好的密实填充作用,降低了水泥的孔隙率,有效阻止了侵蚀介质的进入,其耐酸腐蚀性能与未掺矿渣和粉煤灰的碱式硫酸镁水泥相比有明显提升,其中,掺矿渣的碱式硫酸镁水泥耐酸腐蚀性能更优。     

Management of arsenic-laden water plant sludge by stabilization

When arsenic is co-precipitated with iron hydroxide in an arsenic-removal water plant, a large quantity of sludge is generated which contains a good amount (an average of 6.1 kg As/m³ of sludge) of arsenic. The present paper attempts to find some suitable solutions to disposal of this arsenic-bearing sludge. As a safe means of disposal, stabilization processes such as briquette production, cement mortar and concrete making have been attempted in the present investigation. It has been found that in the preparation of briquette, cement–sand mortar and concrete, mixing up to 10, 18 and 40% of arsenic-bearing sludge by volume, respectively with the other common ingredients did not produce a TCLP leachate beyond its permissible direct inland water discharge standards of 0.2 g/m³ as stipulated by the Ministry of Environment & Forest, Government of India. Also, it was noted that 25% (by volume) of cement could be replaced by fly ash to stabilize arsenic-laden sludge to the tune of 11% by volume of cement–sand (1 : 3) mortar.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Portland cement-fly ash-silica fume systems in concrete

Laboratory flow, strength, and ultrasnic pulse velocity tests were performed on mortars made with 70% (by weight) of portland cement and 30% of pozzolanic materials where the pozzolanic materials consisted of various combinations of fly ash and silica fume. In addition to these ternary systems, binary blends, such as Portland cement and fly ash, and Portland cement and silica fume, along with 100% Portland cement mortars, were investigated for comparison. The purpose of the investigation, preliminary in nature, was to see under what circumstances, if any, would be a synergistic action when a ternary system of Portland cement-fly ash-silica fume is used in a mortar or concrete.Mortars were made with two cements of type I and two cements of type III along with class F and class C fly ashes. One silica fume was used. Standard flow tests were performed on the fresh mortars, and compressive strength as well as ultrasonic pulse velocity tests were performed with each hardened mortar at various ages up to 28 days. It is expected that the results and conclusions obtained here on mortars will be transferable to concretes.There are several novel, or at least lesser known, results of the investigation. For instance, a new explanation is offered for the plasticizing effect of fly ash which is based on the optimum particle-size distribution concept. Another such result is that ground fly ash produced greater flow increases with type I cement than with type III. A third finding is that the superplasticizer is more effective in increasing the flow as well as strength when the mortars contain fly ash and/or silica fume than in the case of mortars without mineral admixture. Also, it appears that when type I cement is used, the silica fume in the quantity of 5% of the weight of the cement produces relatively greater strength increase in the presence of fly ash than without fly ash.These promising results are preliminary in nature. Therefore, further research is justified with ternary systems in concrete. The presented work is a portion of a larger investigation.     

纳米SiO2与粉煤灰协同改性水泥基材料性能研究

通过调整纳米SiO_2与粉煤灰的比例,研究了两者协同作用对水泥基材料性能的影响。结果表明,纳米SiO_2(NS)和粉煤灰协同作用效果优于NS单一掺加,3%(质量分数,下同)纳米SiO_2和不大于30%的粉煤灰同时掺加可以补偿粉煤灰引起的早期强度降低,且砂浆28d抗压强度不降低。随着NS掺量增加水泥基材料的干燥收缩增大,粉煤灰可以改善纳米SiO_2对干燥收缩的不利影响。随着NS掺量的增加,试件的抗冻性和抗氯离子渗透性能均得到提升,掺加3%NS与30%粉煤灰使水泥基材料达到最佳耐久性能。NS可以缩短水泥水化诱导期,加速水泥水化进程,且使胶凝体系总放热量增加。在水泥粉煤灰体系中掺入NS后,非蒸发水含量在早期明显增多,但在后期增长缓慢。     

Characterisation of ground hydrated Portland cement-based mortar as an additive to alkali-activated slag cement

The sustainable development of cement manufacturing requires extension of the raw material base, including large-tonnage waste. Hydrated mortar waste is a promising mineral resource for the production of Portland cements and alternative binders, such as alkali-activated slag cement. The influences of ground-hydrated mortar aged for 3 months on the properties of alkali-activated slag fresh and hardened pastes were performed. The results show that the properties are dependent on the concentration (2.5–60%), cement:sand ratio (1:1–3) and fineness (200–600 m²/kg) of the ground hydrated mortar; the alkali activator (sodium carbonate and sodium silicate); and the curing conditions (normal conditions and steam curing). The fresh paste properties that we considered in this study included the water requirement and the setting time; the hardened paste properties we considered were the water absorption, the density, and the compressive strength after 2, 7, 14, 28, 180 and 360 days of ageing. The ground hydrated mortar improved the early strength and the long-term strength of the alkali-activated slag paste and replaced the slag up to 50%. The factors that affecting the strength of the alkali-activated slag cement with ground hydrated mortar as an additive were, in order of influence, alkali activator type > curing conditions > cement:sand ratio > ground-hydrated mortar fineness.     

水泥-粉煤灰-硅灰基超高性能混凝土水化过程微观结构的演变规律

超高性能混凝土(UHPC)具有卓越的力学性能和耐久性能,应用前景广阔。采用扫描电镜背散射电子图像、热重法和氮气吸附法系统研究了水泥-粉煤灰-硅灰基UHPC浆体水化过程中微观结构的演变过程。结果表明:UHPC浆体在早期水泥水化较快,但7d后水化变得较为缓慢,粉煤灰在UHPC浆体中反应较为缓慢,28d时反应程度仅为7%;UHPC浆体中Ca(OH)2含量早期上升快速,由于硅灰和粉煤灰的火山灰反应逐渐消耗,3d后含量开始下降,但28d时浆体中仍存在部分Ca(OH)2;此外,在水化过程中,UHPC浆体的比表面积不断降低,孔隙率逐渐下降,水化产物变得更为致密。     

Effects of the addition of nanosilica on the rheology,hydration and development of the compressive strength of cement mortars

This paper examines the rheology, hydration kinetics and development of the compressive strength of cement mortars including nanosilica and fly ash. The contents of these materials and the superplasticizer dosage are related to different rheological and strength parameters. Effects on rheology were analysed through yield stress and viscosity. Calorimetry tests were carried out to assess the variations in cement hydration kinetics, and the maximum and minimum heat release rates were analysed. Compressive strength was evaluated at different ages up to 56 days. The equations presented in this paper make it possible to optimize mortar proportionings that fulfil required performance levels in both fresh and hardened states.     

Properties of concrete made with alkali-activated fly ash lightweight aggregate (AFLA)

An experiment was performed to investigate the properties of the hardened paste of fly ash by alkali activation and to determine the possible use of the paste in the production of lightweight aggregates. The highest compressive strength was 33.9 MPa, for paste with 10% NaOH, 15% sodium silicate, and 5% MnO₂, cured at room temperature after 24 h of moisture curing at 50 °C. The hardened paste of fly ash was granulated to produce AFLA (alkali-activated fly ash lightweight aggregate). AFLA exhibited specific gravity (SSD, OD), water absorption, unit weight, and solid volume percentages of 1.85 (SSD), 1.66 (OD), 11.8%, 972 kg/m³, and 58.6%, respectively. The results of the heavy metals leaching test met US EPA regulations. The concrete using AFLA exhibited a compressive strength of 26.47 MPa and good freeze–thaw resistance at 6.0% entrained air content.     

Effect of gypsum type on properties of cementitious materials containing high range water reducing admixture

In this study, the effect of cement gypsum type on properties of the properties of cement paste, mortar and concrete mixtures containing high range water reducing admixture (HRWR) was investigated. Two different types of cement prepared from the same clinker but containing either calcium sulfate hemihydrate or dihydrate as retarder were used. The fresh and hardened (compressive strength and drying-shrinkage) properties as well as static and dynamic rheological behavior of the mixtures were investigated. Compared to the mixtures containing dihydtate, the fresh and rheological properties of mixtures were negatively affected when cement-containing hemihydrate was used. However, hemihydrate utilization had a positive influence on the early compressive strength. The adverse effects on fresh properties were more significant in paste mixtures. These negative effects decreased in the mortar and concrete mixtures. The presence of hemihydrate in cement was found to increase the drying-shrinkage.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.     

Abrasion resistance and mechanical properties of high-volume fly ash concrete

The abrasion resistance and mechanical properties of concrete containing high-volume fly ash (HVFA) were investigated. Sand (fine aggregate) was replaced with 35, 45, and 55% of Class F fly ash by mass. The water to cement ratio and the workability of mixtures were maintained constant at 0.46 and 55 ± 5 mm respectively. Properties examined were compressive strength, splitting tensile strength, flexural strength, modulus of elasticity and abrasion resistance expressed as depth of wear. Test results indicated that replacement of sand with fly ash enhanced the 28-day compressive strength by 25–41%, splitting tensile strength by 12–21%, flexural strength by 14–17%, and modulus of elasticity by 18–23% depending upon the fly ash content, and showed continuous improvement in mechanical properties up to the ages of 365 days. Replacing fly ash with sand significantly improved the abrasion resistance of concrete at all ages. Strong correlation exists between the abrasion resistance and each of the mechanical properties investigated.     

Limestone addition effects on concrete porosity

The effect on porosity (including absorption and sorpitivity) of cement paste and mortar/concrete, of limestone addition to Portland cement is assessed. Based on globally sourced literature published in English since 1993, consisting of 171 publications from 35 countries. The data analysed were from wide ranging tests. The effect on pore structure was also examined in terms of type of Portland cement and limestone, cement fineness and method of producing it, curing, maturity and water-cement ratio, as well as the cement composites with fly ash, slag (GGBS), silica fume and metakaolin and related to strength. Overall, it is suggested that though the use of limestone up to 25% with Portland cement should not impair the pore structure, limit on limestone content for its effect on strength is likely to be about 15%. This should be considered where higher proportion of limestone content is allowed in the Standards.     

Efficiency,skin strength and sorptivity of fly ash concretes

Four grades of concrete with and without fly ash were devised and tested for compressive strength. The concretes were cured in three different curing regimes. The skin strength of concretes under inadequate curing was calculated by assuming a linear model for the variation of strength, and the strength difference between cement and fly ash concretes has been worked out. The skin strength of cement concretes was found to be higher than that of fly ash concretes. The test results were found to be affected by the size of the test specimen, when proper curing was not provided. The difference in sorptivity of fly ash and cement concretes cured for four days and not provided with any initial curing has been included. For all grades of concrete, the sorptivity of fly ash concrete was found to be marginally higher. The difference in sorptivity between fly ash and cement concretes was observed to increase as the strength of the mix decreased. The effect of initial curing was found to be highly significant. The sorptivity of samples with no curing was twice as much as those with four days initial curing. Besides the material properties, the age and strength of a fly ash concrete were also found to be important factors in determining the cementing efficiency of the fly ash.     

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.     

垃圾焚烧炉渣-粉煤灰复合水泥的性能研究

研究了垃圾焚烧炉渣及粉煤灰单掺和复掺时硬化水泥浆体的力学性能和水化机理,比较了两者的活性,探讨了两者作为辅助性胶凝材料利用的可行性.研究表明:掺有垃圾焚烧炉渣及粉煤友的复合水泥,其强度均有不同程度的下降,它们的掺入在一定程度上延缓了水泥的水化过程,且垃圾焚烧炉渣的水化反应活性稍高于粉煤灰;掺垃圾焚烧炉渣及粉煤灰的复合水泥中重金属离子浸出量小,在等掺20%的条件下,浸出量远低于国家标准,说明在一定的情况下,焚烧炉渣及粉煤灰作为辅助性胶凝材料使用是安全的.